The present invention relates to a light source for an optical head to be employed in an optical disc drive which is capable of read/write data on optical discs according to different standards.
There are a plurality of standards in terms of thickness of a protect layer (a transparent layer covering recording surface) and/or density of recorded data. For example, in a CD (compact disc) or CD-R (CD recordable), the data density of which is relatively low, the thickness of the protect layer is 1.2 mm. In a DVD (digital versatile disc), the data density of which is relatively high, the thickness of the protect layer is 0.60 mm. In order to read/write data on the DVD, the size of a beam spot formed on the data recording surface should be made sufficiently small. For this purpose, a laser beam whose wavelength is relatively short (e.g., 660 nm) should be used. For reading/recording data on the CD-R, in view of the reflectivity characteristics thereof, a laser beam whose wavelength is longer (e.g., 780 nm) should be used.
Therefore, a light source for an optical head which is used for both the DVD and CD-R should be provided with at least two light sources (laser diodes) emitting light beams whose wavelengths correspond to the DVD and CD-R, respectively, and a collimating lens that collimates each of the beams emitted by the at least two laser diodes. Each of the collimated beams is incident on the objective lens. In such a type of the device, if the two laser diodes are independent optical elements, the size of the light source unit becomes too large, which prevents downsizing of the optical head. Accordingly, it is preferable that a multi point emission laser diode be used which is a single element having a plurality of light emission points emitting beams having different wavelengths, respectively.
If the multi point emission laser diode is used, the plurality of light emitting points are spaced from each other in the direction perpendicular to the optical axis. Accordingly, the beams emitted by the respective light emission points and passed through the collimating lens may be different from each other. Therefore, if two light sources for DVD and CD-R are provided, at least one of light beams is incident on the objective lens as an off axial beam. Then, vignetting may occur or a relatively large aberration may occur.